A/n: Here' part 3 everybody, we'll see just how Draco grew to the size he is.
Flashback:
In the morning Draco smiled as he wandered the halls that morning.
Why? Well, he'd attained some rather interesting information.
'A growth spell, eh?' Draco mused. 'I could certainly use that.'
He'd spied on them yesterday, learning how the spell worked, but he did not want to test it just yet. He needed that book first, then action.
He found that very opportunity when he found the mudblood roaming the halls with her bag wide open.
After a minute of stalking, he observed the very same book she used yesterday was there, poking out, so all he needed to do was take it without her noticing.
'Not a hard task when she's in a rush.' Draco thought to himself.
Honestly, he never thought it would be that easy to stalk up behind her and yank the book out of her open bag, but it seemed like she was to oblivious to her surroundings.
"Now that I have the book, we can get some work done." Draco muttered to himself.
'But let's see where the little mudblood's going first, shall we?' Draco thought.
'So that's the effect on a human being.' Draco thought as he stared at the massive form of his rival, Harry Potter. 'I can't wait to show just who's boss.'
"Growkus!" He cast upon himself.
He felt his clothes get ever so slightly tighter, but nothing much more than that.
'What?' Draco thought in frustration.
Skimming through the book again, he found that the process takes time at the beginning, but with another skim he found a spell to speed that up. Before he did that however, he cast a spell on his underwear so it would magically stretch without tearing as he grew, just to be safe. He had some dignity after all, unlike Potter over there.
"Speedormus!" He cast upon himself.
Now this gave the desired effect.
Within seconds a strange sensation flooded through him as he could feel his body magically expand, growing inches in mere moments and feet in mere seconds. His clothe tore themselves off of his body as a six-pac formed on his stomach, his arms and legs growing slightly more toned too as he grew, grew and grew.
"Ngh.. now this is more like it." Draco grinned as he looked to himself in his 17ft tall glory, then back to them.
It seemed that the mudblood was talking about a mushroom that could turn them to normal. No, he couldn't have that, oh no. This power felt incredible and he wasn't quite ready to give it up.
And after Harry spoke, it was time to reveal himself.
"Oh, I don't think so Potter." Draco called out as he stepped towards them. "I can't let you find a cure now, can I?"
A/n: That's how he grew to the size he is now.